My friend offered a deal to sell me his RPK and RPD.

The RPK is a Hesse, and I know that brand has a bad reputation. However, I lubed it and shot it with both my Romy drum mags and some various metal mags and it did just fine.

I have yet to see the RPD, but it's unfired and comes with 7 drum/belt combos.

I don't yet know the brand of RPD.

But, is there anything specific I should look out for on the RPD before I'm 100% committed?

Yes... if it has been dry fired a lot, there is an inner firing pin that can get peened over, causing it to misfire. FYI... I ran into this issue on a wiselite and a project guns. Guy who ran project guns taught me the fix, and sent the parts.